Previously, invisibility devices have been invented for cloaking large objects [1-4] at or below the radio frequency range. There the cloak is a spherical object made with special material. Inside the cloak, there is a hole where the object which is made invisible is placed.
The drawback of these devices is, that it is very narrow band. Because of the narrow bandwidth, it does not work for signals.
Another related study involves reduction of forward scattering from cylindrical objects using hard surfaces [5]. There the wave is guided around the hided object. The device is broad-band, but works only for one angle of incidence. Therefore the radiation source can not be placed near the object which is made invisible. It can be used to hide struts from electromagnetic wave coming from one direction, but it can not be used to construct invisible supporting walls.
As far as we know, no-one has considered the advantage of invisible structure. Invisible structure can work for example as a supporting structure or as a mechanical shield, but still to be invisible for electromagnetic radiation. If an antenna is placed behind such an invisible structure, the radiation of the antenna can pass the structure freely. At the same time, the material can be a supporting structure or it can give a mechanical cover for the antenna. The novel structure is also broad band and it works for signals.
Wires can be placed inside the structure while maintaining the invisibility. For example the mechanical strength of the structure can be increased by adding metallic wires. Also electric wires can be placed inside the structure and still the material is invisible.
How Invisible Structure Works
Invisibility
The invisible structure passes the electromagnetic radiation through freely. It simulates free space, or any material surrounding it. In practice, there is always some un-idealities. Despite of this, the invisibility properties can be optimized for a desired application.
Reflection Free
The invisible structure minimizes the back scattering. This is because the invisible structure can be impedance matched with any surrounding material. For example ordinary window glass does have back scattering. This can be seen as mirror reflections from the window.
Mechanically Strong
The advantage of solid invisible structure is that it can be a part of a bigger construction. At the radio frequency range, materials which have the reflection constant near that of the free space are typically mechanically soft materials and they can not be used as supporting structures for heavy objects. The invisible structure can contain large amount of metallic wires, which makes it stronger than any ordinary material witch wave propagation properties close to air.
Broad Band
The invisible structure works for signals, because it is a broadband device. Real-life electromagnetic signals have always finite frequency band with. That is to say, signals have energy in a continuous range of frequencies. The invisible structure can be designed to work in a desired frequency band with. Then both the transmission line network and the matching layer are matched to work at this frequency band.
Two and Three-Dimensional Realizations
For a special use, the invisible structure can be simplified. Sometimes it might be enough to hide the structure from only one angle of incident and one polarization. In that case two dimensional invisibility is enough.
The invisible structure has two and three dimensional realizations. The three dimensional realization corresponds to three dimensional transmission-line network, which has three dimensional connections. Two dimensional network has connections in a plane.